This invention is related to an impact dot printer and, more particularly, to a print head for an impact dot printer which decreases the sliding resistance of the wires through the guide holes and thereby increases the reliability and durability of the print head.
Impact dot printers are known in the art and are exemplified by Unexamined Japanese Patent Publication No. 7351/1991. Such a print head for an impact dot printer as disclosed is conventional.
FIG. 1 of the present application discloses a print head of a conventional impact dot printer which is characterized by injecting a lubricant 20 of an appropriate viscosity at the back of a front end guide 22. The lubricant prevents infiltration of fine particles of ink, ink ribbon scraps, print medium dust, etc., from guide holes 24 of the front end guide 22. When dirt particles such as ink, ink ribbon scraps and print medium dust enter the guide holes 24, the slidability of print wires 26 is impaired and print wires 26 and guide holes 24 undergo increased wear.
A recent trend in impact dot printing is the development of a growing demand for impact dot printers capable of printing on various types of printing papers such as postcards and labels, This has increased the need and accelerated the development of printers that receive printing paper through horizontal insertion. In other words, these new trends have encouraged the development of impact dot printers where the printing paper is inserted horizontally and the printing paper is not required to be bent around a platen as in conventional dot matrix printers. In other words, the print head 30 of a conventional dot matrix printer is mounted in an upright direction to print on paper 29 bent around cylindrical platen 31 as shown in FIG. 2A. On the other hand, the print head 30' is mounted in a face down direction so that a print sheet 29' is laid horizontally on flat platen 31' as shown in FIG. 2B.
FIG. 1 discloses a print head and specifically an expanded view in the vicinity of the tips of wires 26. A wire dot print head of the type disclosed usually operates by print wires 26 directly contacting ink ribbon 32. Ink ribbon 32 is maintained in close proximity to front surface 28 of front end guide 22 and the print wires 26 is driven through opening 33' in mask 33 to strike paper 29' on platen 31'. When the print head is in a standby position, the wires 26 are maintained approximately 0.01 millimeters projected outward from front surface 28 of front end guide 22. Problems occur when the print head 30 is mounted in a face down direction as shown in FIG. 2B for a long period of time. As shown in FIG. 1, the lubricant 20 seeps through guide holes 24 and seeps through clearances or gaps between the front end guide 22 and nose 23 due to the specific gravity of the lubricant used. As a result of lubricant 20 seeping through guide holes 24 and the clearances discussed above, lubricant 20 becomes transferred onto ink ribbon 32.
The lubricant 20 that contacts ink ribbon 32 causes some ink components of ink ribbon 32 to be lost. After the ink components on the ink ribbon 32 have been lost, the operation of the ink ribbon 32 is impaired. In other words, when the wire 26 contacts ribbon 32 in an area where lubricant 26 has been transferred thereon, an ink dot cannot be formed by the contact of wire 26 on lubricant transferred portion 34 of ink ribbon 32 as shown in FIG. 3. Accordingly, when wires 26 contact lubricant transferred portion 34 of ink ribbon 32, ineffective printing occurs as shown by portions 36 of FIG. 4.
Print heads 30, which are mounted in an upright position as disclosed in FIG. 2A, can also suffer from the same deficiencies as those mounted in a face down position because lubricant 20 can leak out of the print head through guide holes 24 when the amount and viscosity control of the lubricant 20 injected at the back of the front end guide 22 is improper. This occurs when the viscosity is too low or there is too much lubricant injected behind the front end guide 22, thereby causing increased pressure and lubricant leakage. Under these circumstances, a print head mounted in the upright position of FIG. 2A suffers similar defects to that shown as portion 36 of FIG. 4.
As a result of the problems disclosed hereinabove regarding leakage of lubricant through the print head, the manufacturing cost of print heads has increased because strict control over the viscosity and amount of lubricant injected into the back of the front end guides of the print heads is required, even if the head is mounted in the upright position. Furthermore, when the print head is mounted in a face down position as disclosed in FIG. 2B, the prior art devices have been required to stop injecting lubricant at the back of the front end guide completely, in order to prevent defective printing as described hereinabove with regard to FIGS. 3 and 4. As a result of eliminating the use of lubricant, there have been noticeable losses in reliability, durability and printing quality. These losses are due not only to wear of the guide holes of the front end guide, but also to increases in sliding resistance of the wires due to ink, paper dust and ribbon scraps infiltrating into the guide holes and due to picking up of the ribbon by the wires.
Accordingly, it is desirable to provide a print head for an impact wire dot printer wherein the print head can be mounted in either an upright or a face down position, and lubricant can be injected at the back of the front end guide to prevent defective printing. Further, it is desirable to decrease the cost of production of the print head by decreasing the degree of monitoring required with regard to the viscosity and amount of lubricant injected at the back of the front end guide.